In an optical-disc of the type presently used with a DAD (Digital Audio Disc) player or a video disc player, audio or video information is recorded in optically coded form as a series of recesses or pits which are formed in the information-carrying face of the optical-disc and arranged in either a spiral track or a plurality of concentric tracks about the center axis of the optical-disc. The audio or video information thus recorded is reproduced by optically scanning the individual recesses or pits along the spiral track or the concentric tracks by means of a beam of laser light which detects the lengths and spacings between the scanned pits. During playback, the optical-disc is usually rotated about the center axis thereof at a constant linear velocity or a constant angular velocity, and the beam of laser light is displaced radially relative the optical-disc by a tracking device or a pick-up unit which is a part of the DAD or video player. The laser beam is directed to a target track and is reflected by the information carrying face of the optical-disc or is passed through the optical-disc. The beam of light thus reflected or passed through the optical-disc is then converted into an electric signal by means of a photoelectric transducer unit mounted on the pick-up unit so as to facilitate further conversion into an audio or video signal.
In order to reproduce the information recorded on the optical-disc, it is necessary that the laser beam precisely track the target information track on the optical-disc. To this end, optical-disc information reproducing apparatus has heretofore employed a method wherein a light beam, exclusively used for tracking, is generated separately from the light beam used for reproducing the audio or video information signal, and a photodetector for detecting the tracking light beam is used to effect tracking control. This method has a disadvantage, however, in that the construction of such an apparatus is complicated. In cases where the use of a single light source is intended, the employment of a plurality of light sources being expensive, it is necessary to form three light beams for picking up the audio or video signal, focusing and tracking. This brings about a further disadvantage in that the power of such a single light source must be very high.
A method has also been employed wherein the audio or video information signal, the focusing signal and the tracking signal are all extracted with a single light beam. According to this method, the photoelectric transducer unit includes a plurality of photodetectors, for example, four photodetectors. An image of the pit formed by the aid of the single light beam is thus converted into the respective electric signals by the four photodetectors. The signals from the four detectors are then converted into three signals carrying audio or video information, focusing information and tracking information. The tracking information carrying signal is produced based on inconsistencies in the electric signals detected by the four photodetectors whenever the light beam wanders from the target track of the optical-disc.
This method is advantageous in that the apparatus is kept relatively simple and, since the three information signals are obtained with a single light beam, the light source may be of low power. However, the method does have a disadvantage when implemented in connection with optical-disc information reproducing apparatus. In such a case, the tracking information carrying signal must be independent from the other signals in order to accurately indicate the location of the target track. The tracking information carrying signal is, however, apt to be distorted by the audio or video signal due to, for example, lack of uniformity among the four photodetectors as to their transducing characteristics and/or their relative mechanical arrangement. The distortion of the tracking information carrying signal causes inaccurate operation of the tracking servo system for maintaining the light beam on the target track so that reproduction of the audio or video signal becomes impossible.
Referring now to FIGS. 1 and 2, description will now be made of a typical prior art apparatus. FIG. 1 is a diagram for explaining the tracking servo system in a prior art optical-disc information reproducing apparatus. In the figure, parts which are not relevant to the control of the apparatus are omitted from the illustration. A single light beam 10 emerging from a laser light source (such as a He-Ne laser) 12 passes through a beam splitter 14. Beam 10 converges at a point on an optical-disc 16, termed the convergent spot 18, by means of an objective lens 20. Optical-disc 16 is rotated at a constant linear velocity by a motor 22. On optical-disc 16 are provided information tracks 24 bearing information relating to audio signals or video signals, etc., which have been recorded in coded form at high density, and which information is to be read at convergent spot 18.
At convergent spot 18 of light beam 10, the information of information track 24 is read in the form of changes in the reflection factor of the light due to a series of recesses or pits representing the coded information of the audio or video signal. The reflected light returns to objective lens 20 and is separated from the entrance beam by beam splitter 14. The reflected light is then guided to photodetector 26 such that light beam 10, having reached photodetector 26, becomes a detected spot bearing image 28 of the pit.
Photodetector 26 is divided into four individual photodetection portions corresponding to four regional elements D1, D2, D3 and D4 that are divided by boundary line L1 longitudinal to an image of the tracking direction and by boundary line L2 perpendicular to the image of the tracking direction. Pit image 28 is placed on or displaced from longitudinal line L1 according to light beam 10 tracing target track 24 accurately or inaccurately, respectively. Individual photodetection elements D1, D2, D3 and D4 output, respectively, electric signals S1, S2, S3 and S4 varying in accordance with the areas of elements D1, D2, D3 and D4 overlapped by pit image 28.
Signals S1, S2, S3 and S4 are matrixed by a matrix circuit 30 and then differently combined into five signals: audio or video information carrying signal A1, tracking information carrying signals T1 and T2, and focusing information signals F1 and F2. Signal A1 is formed by the addition of all of detection signal S1 to S4. Signal T1 is formed by the addition of detection signals S1 and S3 from elements D1 and D3, which are orthogonal to each other and thus form a combination obliquely disposed to the track image. Signal T2 is formed by the addition of detection signals S2 and S4 from elements D2 and D4 which are orthogonal to each other and form a combination intersecting the former combination and disposed obliquely to the track image. Signal F1 is formed by the addition of detection signals S1 and S2 from elements D1 and D2 which are adjacent each other across perpendicular boundary line L2. Signal F2 is formed by the addition of detection signals S3 and S4 from elements D3 and D4, which are also adjacent each other across perpendicular boundary line L2.
Focusing information carrying signals F1 and F2 are applied to a focusing control section (not shown). Audio or video information carrying signal A1 is applied to an output terminal 32 through a HPF (high pass filter) 34 and an amplifier 36, and is used for audio or video signal reproduction. Audio or video information carrying signal A1 appearing on output terminal 32 has a waveform b varying in accordance with light beam 10 moving across tracks 24a, 24b and 24c, as shown in FIG. 2. Respective tracking information carrying signals T1 and T2 are applied to a differential amplifier 38. Output signal T3 of differential amplifier 38 has a waveform c varying, as shown in FIG. 2, in accordance with the above-mentioned movement of light beam 10. Signal T3 is applied simultaneously to input terminals of sample-and-hold circuits 40 and 42.
Signal A1 on output terminal 32 is applied simultaneously to a leading edge detecting circuit 44 and a trailing edge detecting circuit 46. Output signals A2 and A3 from leading edge detecting circuit 44 and trailing edge detecting circuit 46, respectively, have waveforms d and e, as shown in FIG. 2. Impulses in waveform d appear when positive-going portions of waveform b increase over a reference level, usually a ground level. On the other hand, impulses in waveform e appear when negative-going portions of waveform b decrease below the reference level. Signals A2 and A3, thus comprising impulses representing the leading edges and trailing edges of signal A1, are applied to trigger terminals of sample-and-hold circuits 40 and 42, respectively. Accordingly, signal T3 (waveform c) is individually sampled according to the impulses of respective signals A2 and A3, and is held in the respective instant sampled levels so that signals T4 and T5 with waveforms f and g appear, respectively, on output terminals of sample and-hold circuits 40 and 42, as shown in FIG. 2. Signals T4 and T5 are applied to a differential amplifier 48. Output signal T6 from differential amplifier 48 has a waveform h and is used as a tracking error signal. Tracking error signal T6 is applied to a motor 50 through an amplifier 52 for controlling tracking movement of objective lens 20.
Tracking signal T6 becomes negative or positive, respectively, when light beam 10 deviates from target track 24b in either direction, i.e. upwardly or downwardly as shown in FIG. 2, and the absolute value of tracking signal T6 becomes maximum when light beam 10 is between adjacent tracks 24a and 24b or 24b and 24c. Accordingly, motor 50 is able to move objective lens 20 under the control of tracking error signal T6 so as to cause light beam 10 to approach target track 24b from either side of track 24b, as shown in FIG. 2.
The prior art apparatus described above, however, has several disadvantages. That is, signal T3 on the output terminal of differential amplifier 38 is obtained from electric signals S1 to S4 delivered by the four photodetection elements D1 to D4. Signal T3 is distorted in its level when an undesired phenomenon occurs; for example, a change or fluctuation of strength of light beam 10 influences signal T3. Furthermore, deformations and/or optical noise in the optical system or an unequality among the four photodetection elements D1 to D4 may also cause distortion of signal T3. Such distortion of signal T3 causes tracking error signal T6 to drive motor 50 erroneously. Further, there are relatively long periods of time, P1 and P3, during which signal T6 gradually decreases. Signal T6 during these periods P1 and P3 cannot be used for tracking control of light beam 10 on target track 24b. Only during period P2 in which signal T6 gradually increases is signal T6 able to be used for tracking control of light beam 10 towards target track 24b. Thus the period during which tracking control of light beam 10 to target track 24b is effective is short and restricted. Additionally, because circuits 40, 42, 44 and 46 for edge detections and sample-and-hold operations must exhibit high speed response characteristics, the apparatus becomes complicated in construction and thus expensive.